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Tic-Tac-Toe
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Three in a row on a 3×3 grid — the game everyone learns first. Simple enough to explain in ten seconds, and solved so completely that two careful players will draw forever.

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How to play
  1. Players take turns placing their mark on a 3×3 grid. One of you is ✕, the other is ◯.
  2. The first player to get three of their marks in a row — horizontally, vertically or diagonally — wins the game.
  3. If the grid fills up with no three in a row, the game is a draw.
  4. Play a single game or a best-of match. Between games the starting player alternates, so nobody keeps the first-move advantage.

Strategy & tips
  • With perfect play it is always a draw. This isn't an opinion — the game is completely solved. Winning means waiting for your opponent to make a mistake, not out-playing them.
  • Take the centre first. It belongs to four winning lines, more than any other square on the board. Corners belong to three, edges to only two.
  • If they take the centre, answer with a corner. Replying on an edge gives your opponent more ways to build a threat.
  • The real weapon is a fork. That's a single move creating two threats at once — your opponent can block one, but not both. Almost every win at this game is a fork, and almost every loss is failing to see one coming.

Where it comes from

Tic-Tac-Toe is ancient. A version called terni lapilli was scratched into stone by Romans, and the modern grid has been used for centuries as a first lesson in strategy. It's also a milestone in computing history: OXO, written in 1952, was one of the earliest video games ever made, and the game is a standard teaching example for the minimax algorithm.